As the femme fatale for whose love a king renounced his throne, the universally reviled Mrs Simpson must be thanked for saving the nation – better make that the Empire, which was still around in 1938 – from being ruled by a man described by Lord Wigram, George VI's private secretary, as almost certainly certifiable. We all know that the Prince of Wales was madly in love with Wallis, but until Sebba's revelations about his permanent condition of mental and physical pubescence, I didn't know the half of it.

Constance Spry, who did the flowers for the Windsors' wedding in France, remembers him kneeling on the floor to read damp copies of old Times newspapers under the uncut roses – but do Edward's lack of interest in literature and music, and his complete lack of body hair, really make him mad? Sebba has admitted that, unlike the scores of books, films, documentaries and even Wallis's own memoirs, she wanted to make Mrs Simpson more sympathetic. Well, she hasn't.

Wallis, née Bessie Warfield from Baltimore, remains the ruthlessly ambitious, gold-digging socialite portrayed on screen by, among others, Faye Dunaway, Joely Richardson and Gwyneth Paltrow. It's hard to feel sorry for a woman this tough. As for her legendary sexual domination of "the little man", as all his mistresses referred to him, until the "China Dossier" – said to detail how Wallis had learned techniques variously called the Baltimore grip, Shanghai squeeze or China clinch when living in the Far East – we could only guess what went on in the master bedroom. "The great ignorance in sexual matters in early 20th century in middle and working-class Britain is key to understanding the story of Wallis, why she was attacked so fiercely at the time and why she has since become such a talisman for gay and lesbian minorities … her struggle is emblematic of a wider struggle for greater sexual freedom," writes Sebba.

She may be right. The Technique of Sex, the first manual on the subject, published in 1939, sold half a million copies in hardback alone and remained in print for 50 years. The publishers stipulated that the book's illustrations had to be bound and sealed in a separate packet at the back as they were "of interest only to the serious reader".

This is an absolutely cracking story all but ruined by lacklustre narrative and the wrong reader. For all that, I still recommend you to listen to Say's epic adventure. It started in January 1939 when, aged 19, she left London to work as an au pair in Avignon, failed to get back home before the Germans invaded France, was interned as an enemy alien in Alsace, escaped through France and Spain, eventually making it to Portugal, where she was repatriated. She died in 1996 leaving the incomplete manuscript for her daughter to finish. Sadly, much as I love her warm, Aga saga voice, Phyllida Nash is not a gawky 19-year-old girl running for her life. But you can't keep a good story down.

When Carrie Fisher was 19 she played Princess Leia in Star Wars. Forty-eight years on, in a voice raddled with experience, excess, exposure and exhaustion, the daughter of Eddie Fisher and Debbie Reynolds and ex-wife of Paul Simon muses on abuse, abandonment and addiction – and it's hilarious. That's show biz.